I've created a new WIKI page at http://ironruby.rubyforge.org/wiki/wiki.pl?TowardsRails to summarize the current progress towards executing Gems and Rails. It contains a priority list of unresolved bugs, unimplemented methods and uncommitted patches. It's certainly not a complete list, just those issues that I've explicitly encountered so far.
Anyone looking for a method to implement or a bug to fix could certainly look here.
Feel free to edit, append and further annotate it as appropriate and put your name against methods on the list.
Cheers, Wayne.